Manufacture of low-carbon chrome steel



Patented Dec. 16, 1930 UNITED STATES PATENT OFFICE FRANK O. KICHLINE, LEBANON, PENNSYLVANIA, ASSIGNOR TO BETHLEHEM STEEL COMPANY No Drawing.

MANUFACTURE OF LOW-CARBON CHROME STEEL Application filed April 20, 1926. Serial No. 108,377.

This invention relates to the manufacture is replaced in the pig ironbythe metals reof ferro-chromium alloys and is more'particularly directed to the manufacture of chrome steels of low carbon content and of 5 chrome irons containing a substantial proportion of chromium.

The desirable rust-resisting property of low carbon chrome steel and chrome iron is well known, but hitherto the high cost of 10 manufacture, due to the necessity of using ferro-chromium of very low carbon content for alloying with low carbonsteel, has prevented the Wider use of such compositions. When ferro-chromium is obtained by the carbo-reduction of chrome ores, it is impossible to avoid the inclusion of carbon in the ferroalloy, and this residual carbon is normally removed by subsequent refining operations,

Which involve losses in chromium, as for ex-.

ample by bessemerizing the alloy. Ferrochromium of low carbon content may also be obtained by well known alumino-thermic methods, but the cost of the thermic reagent is-prohibitive in a ferro-alloy for use in a commercial steel.

An obj ect of this invention is, therefore, a method of manufacture, whereby low carbon chrome steel or chrome iron may be obtained without having recourse to expensive refining operations, nor to expensive raw materials. a Y

A further object of this invention is the direct utilization of pig iron, Whether itself containing chromium or not, with chrome ore for the production of chrome steel and iron.

A further object .of this invention is to effect the decarburization of pig iron, of steel or of ferro-alloys by the addition or ore inamore eifective manner than has hitherto been attainable.

With these objects in view, the invention consists in reducing chrome ore with the carbon normally contained in blast furnace pig iron, in such a manner that an intimate contact is eifected between the metallic oxides of the ore and the carbon of'the pig iron, so that the carbon at a suitable temperature, such as may be obtained in an electric furnace, reduces the metallic oxides of the ore in substantially equivalent proportions and duced from the ore;

Heretofore, attempts to introduce an alloy- I ing element, as for example chromium, into a bath of steel by the addition of an ore of the alloying element, have not been successful. Operating difliculties are introduced when the ore addition amounts to a economically substantial proportion of the total furnace charge, as would be the case if ahigh content of the alloying element were desired. With chrome ore these difiiculties are principally due to the fact that the chromic oxide of the 1 minuted condition. The pig iron may be ob-' tained in a suitable state of comminution, by

granulating it from its molten condition in the well known manner; while the ore may be crushed to the size desired in any convenient manner. I

Preferably, by rapidly cooling a spray of molten metal. This granulation may be carried out directly when tapping a blast furnace charge, or the pig iron may be melted in a special cupola and granulated therefrom. In order to insure constant conditions in the subsequent furnace operations, it may be advantageous to classify the granules in batches of approximately similar particle size. I- also crush the chrome ore to a size that will allow of its intimate mixture with the granulated pig iron. A furnace charge is made up by mixing a convenientquantity of the granulated- I granulate molten pig iron I with the necessary quantity of lime. This charge is melted in an electric furnace, the temperature of which is maintained at a point suitable for the reduction of chromium and at the end of the heat I obtain a chrome steel, having a content of chromium commensurate with the carbon originally present in the pig iron, and which is itself substantially carbon free. It will be understood that the refining operations for removing undesirable impurities from steel, normally practiced in a basic electric furnace under a lime slag, may be carried out if desired, subsequently to the reducing and decarburizing operation above described. Obviously in order to avoid the necessity of an extensive refining of the fusion bath, and consequent loss of time thereby, I prefer to utilize for my furnace charge a pig iron having a low content of these impurities.

For the purposes of my invention, I have found that pig iron derived from chromiferous iron ores, as for example the Mayari oresof Cuba, are particularly suitable, since the chromium content of the pig iron can thereby be profitably utilized, serving to increase the content of chromium in the alloy above that obtainable by the carbo-reduction of the chrome ore. In the same manner, by using a pig iron containing another alloymgelement besides chromium, as for example nickel, I can introduce this second alloying element into the final product in a simple and economical manner.

The following example will illustrate in a comprehensive manner the practice of my invent-ion in a case where it is desired to manufacture a low carbon chrome steel, containing also an appreciable percentage of nickel. The known methods of manufacture would involve the reduction of chrome ore and the subsequent laborious decarburizationof the ferro-chromium obtained, or alternatively the expensive alumino-thermic reduction of chrome ore; in either case a low carbon ferro-chromium, preferably as low as one tenth of one per cent. carbon, is essential if a low carbon alloy steel is desired. For the same reason, carefully selected low carbon steel scrap would be melted to form the fusion bath, and after the usual refining steps for phosphorus and sulphur the necessary quantity of the aforesaid low carbon ferro-chrome would be added to the bath and alloyed therewith, after which nickel would be introduced by an appropriate addition of nickel plaquettes.

In my process, I take, for example, a chromiferous pig iron, such as that derived from the Mayari ores of Cuba, having the following anaylsis: carbon 3.99, manganese 0.79, phosphorus .073, sulphur .014, silicon 0.40, nickel 0.89 and chromium 2.72. I granulate this pig iron by melting it and pouring the molten stream into cold water, and select sixty pounds of the granulated material of approximately similar particle size. I mix with the granulated iron, one hundred pounds of chrome ore, having a chromic oxide content of about 43%, and crushed to suitable mesh, and also sixty pounds of lime. This mixture is then charged into an electric furnace and melted, the necessary additions of fluorspar being made as required. The reaction between the carbon of thepig iron and the oxides of the ore proceeds smoothly and without involving the production of excessive, nor of viscous, slag. The product obtained has the following composition: carbon 0.14, manganese 0.08, phosphorus 0.044, sulphur 0.015., silicon 0.014, nickel 0.94, chromium 12.39. I

From the above it will be evident that I have devised a cheap and effective process for the production of low carbon chrome steel, without having recourse to highly priced raw materials nor to expensive refining operations. A low carbon chrome steel with a content of from 10 to 16% chromium is well known to possess rust resisting properties and may by my process be readily obtained from any low phosphorus pig iron having a carbon content of 4% or thereabouts. By using a substantial excess of chrome ore and by disintegrating both the pig iron and the chrome ore,,so that they may be intimate- 1y intermixed before being exposed to reduction temperatures, I am able to effect a very complete decarburization of. the iron.

The degree of reduction of chromium from its ore and consequently the percentage thereof finally alloyed with the steel will obviously depend on the carbon introduced by the pig iron. When a chromiferous pig 1IOI1 1s used, the final chromium content will be increased accordingly, similarly a pig iron having a higher carbon content than that above mentioned will be susceptible of being used for the manufacture of low carbon chrome steel and chrome iron having a higher content of chromium than that specified above, or to the manufacture of 'lowcarbon ferrochromium. In such cases, the chromium reducible from the ore to be added to the pig iron, being limited by the carbon content of the latter, the additional chromium desired in the fihal product must be introduced by the pig iron. This involves a supply of highly chromiferous pig iron, that is to say, richer in chromium than that obtainable in the blast furnace from natural chromiferous iron ores.

The reduction of chrome ore in a normal blast furnace for the purposeof obtaining ferro-chromium in which the relative proportions of iron and chromium are approximately those in which they exist in natural chromite is not economically possible. This is partly due to the fact that the reduction temperature of chromic oxide exceeds the normal hearth temperature of the blast furthe carbon of the pig iron is required in my process, a pig ironthat is highly chromiferous maybe obtained from the blast furnace by including in the furnace burden a certain proportion of chrome ore. A pig iron of this description may therefore be utilized as the base of an electric furnace charge for a process such as I have already described, contributing thereto as much as or even a greater proportion of chromium than that rendered available by decarburizing the pig iron with crushed chrome ore in the manner aforesaid.

Such methods of increasing the chromium content in the final alloy are entirely within the scope of my process, in view of the methods well known to the art for the reduction of chrome ore mixed with iron ore in the blast furnace. I

It will be obvious to those skilled in the art, that-my. invention is susceptible of vari-' ous changes and modifications without departing from the spirit thereof and-I desire thereforethat only such limitations shall be placed thereupon as are imposed by the prior art or as are specifically set forth in the appended claims.

carbon 0 Having thus described my invention, what:

I claim as new and desire to ters Patent is:

1. A process for the manufacture of low carbon chronic steel which consists in, intimately mixing comminuted pig iron containing carbon withcrushed chrome ore and slag forming material, and fuzing the mixture in an electric furnace, so as toefli'ect a reductidn of chromium from the ore through the influence of the carbon in the pig iron.

2. A plrocess for the manufacture of low rome steel which consists in, granulating molten pig iron containing carbon, mixing the latter with crushed chrome ore and lime, the ore being inexcess of the carbon requirement, and-fuzing the mixture in an electric furnace, whereby the pig iron form ing the base for the carbon free chrome may be substantially decarburized.

3. A rocess for the manufacture of low secure by Let- I carbon c rome steel which consists in, rapidly cooling moltenpig iron by granulating it in water, so as to conserve its contained carbon in combination with the iron, crushing chrome ore, effecting an intimate mixture of the granulated pig iron, the crushed ore and a Therefore, when a higher alloy 7 suitable slag producer, fu zing the mass in an electric furnace to reduce chromium, from the ore by the carbon in the pig iron, and refining the'resulting bath of alloy steel in the normal manner.

4. Aprocess for the manufacture of low carbon chrome steel which consists in, intimately mixing comminuted pig iron containing carbon with chrome ore and lime, submitting the mixture to a suitable reducing temperature in an electric furnace, whereby chromium in the ore may be reduced by the carbon in the iron, and refining the resulting bath of chrome steel under a lime slag in the normal manner.

5. A process for the manufacture of alloys of iron and chromium substantially low in carbon which consists in, reducing a mixture of iron ore and chrome ore ina blast furnace to produce a highly chromiferous pig iron, granulating the reductionproduct, mixing it intimately with a further quantity of crushed chrome ore and with slag forming material, charging said mixture in the solid phase into an electric furnace, maintaining therein a temperaturecapable of promoting the oxidation of the carbon present in the reduction product by the oxides of the ore added thereto, whereby the carbon in said reduction product may be substantially replaced by chromium from the added ores.

6. A process for the manufacture oflow carbon' steels containing ch omium and another alloying element whic consists in, re-' ducing iron ore containing the second alloying element in ablast furnace so as to include thiselement in the pig iron, granulating the pig iron, mixing therewith crushed chrome'ore and lime, charging the mixture in the solid .phase into an electric" furnace,

mantainin therein a temperature suitable for the car q-reduction of chromlum from its ore and refining the resulting bath of alloy steel in the normal manner.

7. A process for the manufacture of low carbon high chromium iron alloys, which comprises mixing crushed chrome ore and slag forming material with a finely divlded blast furnace'product containing a reduclng agent in substantial amounts and fusing the mixture. p

In testimony whereof I hereunto aifix my signature this 14 day of April, 1926.

- v FRANK O. KICHLINE; 

